This proposed work is in a interdisciplinary, interinstitutional research project involving members of the Biomedical Engineering Laboratory of Rice University; the Hematology Section, Department of Medicine, Baylor College of Medicine; and the Division of Hematology, Department of Medicine, The University of Texas Medical School. The objective of the project is to determine the effects of physical stresses on platelets. Preliminary studies have shown that relatively low (50 dynes cm squared) shear stresses stimulate the release reaction and subsequent aggregation, and have raised the possibility that thrombus formation could result from the effects of this physical force on circulating platelets. The proposed work will be to stablish the effects of exposure time to known, uniform shear fields, the complication of cell-cell interaction--including studies in whole blood, an the related cell-solid surface interactions.